1. Field of Invention
The present invention relates to a die-bonding method of a light emitting diode (LED) chip and an LED, and more particularly to a die-bonding method capable of low-temperature die-bonding and obtaining high-temperature intermetallic layers as well as an LED having a die-bonded structure.
2. Related Art
The technology of adhering an LED chip to a lead frame has been developed for many years. Die-bonding materials are approximately divided into two categories: one is high molecular conductive glue materials, and the other is metal welding materials.
The first category can be seen in ROC Patent No. 463394 entitled “CHIP-TYPE LED AND MANUFACTURING METHOD THEREOF”. The method mainly includes: plating silver paste on a surface of a metal substrate, forming a plurality of lead frames after etching, die-bonding one end of the lead frame and connecting it to the opposite end by wire bonding, performing glue sealing and dicing so as to form a chip-type LED, in which lead frames exposed at the bottom form electrical contacts. In such practice, if glue is not spread uniformly in the bonding process, the die will not be fixed at the preset position, thereby influencing the luminous efficiency. Next, in such die-bonding method, since the high molecular material has extremely low heat resistance, the silver paste bonding layer is easily deteriorated in operation at a high temperature. Further, since the high molecular material has low heat conductivity, the LED die cannot obtain a desirable heat dissipation effect due to the low heat conductivity (the heat conductivity coefficient of the silver paste is only 1 W/M-K). The life and photoelectric conversion efficiency of the LED die are reduced as well.
The second category can be seen in ROC Patent Application Publication No. 200840079 entitled “DIE-BONDING MATERIAL AND METHOD OF LED PACKAGE”. The die-bonding method used in the patent application mainly adopts eutectic bonding based on the metal material of a substrate. First, a layer of eutectic bonding material in an appropriate range is coated on an upper surface of the metal substrate of the package structure. Then, an LED die is disposed on the eutectic bonding material of the substrate. The finished product passes through a hot plate, an oven, or a tunnel furnace to have an appropriate temperature, so as to accomplish the eutectic bonding. This technology employs the eutectic bonding material, and forms a bonding layer of a metal material, and thus achieves better heat dissipation and heat resistance than the silver paste. A part of the eutectic bonding material employed in this patent technology has a high melting point, so that thermal stress easily remains on the LED die in bonding, which damages the die. Although the other part of the eutectic bonding material is a low melting point alloy, after bonding of such bonding material is completed, if the LED is used in an environment of 70-80° C., the bonding layer will be softened, and the contact reliability is greatly impaired.
In addition to the above technologies, US Patent Application Publication No. 2007/0141749 has disclosed introducing ultrasonic waves in the die-bonding process and ionizing the bonding surface by the ultrasonic waves, so as to lower the heating temperature and reduce the thermal stress. This method requires the addition of ultrasonic equipment, which increases the manufacturing cost. Meanwhile, if the ultrasonic waves are operated improperly, the LED die may be vibrated directly to be cracked.